Spatio-temporal patterns and covariance structures of soil water status in two Northeast-German field sites

Spatio-temporal patterns of soil moisture status highly affect the heterogeneity of soil water and solute transport and leaching of chemicals to the groundwater. In order to quantify and describe spatial variability of ecologically highly relevant spatial and temporal processes linked to soil moisture at the land surface, the spatio-temporal covariance structure and the reasons for its change in time need to be identified. Therefore, soil water pressure head was monitored in two surface horizons between April and November 1995 at two field sites with a shallow ground water table, a sandy loam and a heavy clay soil in north-east Germany. For the 10- and the 30-cm depth of the sandy loam soil and for the 30-cm depth of the heavy clay soil the variance of soil water pressure head h (on log basis) was large under wet conditions. With decreasing soil water pressure head the variance of log10(−h) decreased to a critical value, for which a spatial correlation structure disappeared. With further drying, the variance of log10(−h) increased again, and a spatial range of correlation existed. During drying, temporally stable variation patterns developed at both field sites. The change of variance of log10(−h) which is probably associated with changing degree of heterogeneity of flow conditions validates the findings of Roth (1995) obtained from stochastic flow model calculations. In general, the temporal correlation length was better defined than that of spatial correlation. At both depths, correlation lengths of the sandy loam were larger than those of the heavy clay soil. Random spatial variation of log10(−h) occurred under conditions when the hydraulic gradient was close to zero. With our experimental design we could identify (i) temporal persistence of spatial patterns and correlation ranges of soil water status that can be used for representativity studies, (ii) the within-site variation of land surface moisture status, and (iii) a basis for description of spatial processes of effective soil properties that are linked to soil water status at the land surface.